Train your first Convolutional Neural Network for classifying news articles

Natural Language Processing Jun 9, 2021

Convolutional Neural Networks or simply convnets, are generally used for Computer Vision problems with their ability to operate convolutionally, extracting features from local input patches and allowing for representation modularity and data efficiency and coupled with augmentation techniques, convnets can extract a lot of information from a small representative dataset.

The same properties can also help convnets learn representations on sequence data, notably on text. 1-D convnets can be competitive with RNNs on certain sequence processing tasks, usually at a cheaper computational cost.

Patches in 1-D CNN

1-D convnets can recognize local patterns in a sequence since the same transformation is performed on every patch,  a pattern learned at a certain position in a sentence can later be recognized at a different position, making 1-D convnets translation invariant.

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Let's get into the tutorial!

In this tutorial, we will train a model of 1-D convnet layers on BBC News Dataset that maps news articles to the categories they come under. We will also be using GloVe embeddings for tokenization. We will build the model on Keras and train it on Kaggle.

Step 1: Import and Preprocess the dataset

def load_data(filename,encoding):
    data = pd.read_csv(filename,encoding=encoding)
    return data

data = load_data('../input/bbc-fulltext-and-category/bbc-text.csv','latin1')
words = set()
vocab = {}


token = data['text'][0].split()
table = str.maketrans('','',punctuation)
tokens = [w.translate(table) for w in token] 

tokens = [word for word in tokens if word.isalpha()]
tokens = [word for word in tokens if len(word)>2]

data['category'] = data['category'].astype('category').cat.codes
def clean_and_get_tokens(doc):
    tokens = doc.split()
    table = str.maketrans('','',punctuation)
    tokens = [w.translate(table) for w in tokens]
    tokens = [word for word in tokens if word.isalpha()]
    tokens = [word for word in tokens if len(word)>2]
    return tokens

documents = data['text']
for doc in documents:
    tokens = clean_and_get_tokens(doc)
    for token in tokens:
        if token in vocab:
            vocab[token] += 1
        else:
            vocab[token] = 1

for word in vocab:
    if vocab[word] > 5:
        words.add(word)

Step 2: Split the dataset into Training and Testing set

def create_train_test_sets(data,split):
    np.random.seed(0)
    mask = np.random.rand(len(data)) < split
    train_data = data[mask]
    test_data = data[~mask]
    return train_data,test_data

train_data,test_data = create_train_test_sets(data,0.8)

train_documents = []
for doc in train_data['text']:
    tokens = doc.split()
    final_tokens = []
    for token in tokens:
        if token in words:
            final_tokens.append(token)
    final_string = ' '.join(final_tokens)
    train_documents.append(final_string)

test_documents = []
for doc in test_data['text']:
    tokens = doc.split()
    final_tokens = []
    for token in tokens:
        if token in words:
            final_tokens.append(token)
    final_string = ' '.join(final_tokens)
    test_documents.append(final_string)

Step 3: Use Tokenizer to convert sequences of words into encoded text

tokenizer = Tokenizer()
tokenizer.fit_on_texts(train_documents)
encoded_docs = tokenizer.texts_to_sequences(train_documents)

max_length = max(([len(s.split()) for s in train_documents]))
labels = train_data['category']
train_labels = labels
Xtrain = pad_sequences(encoded_docs, maxlen=max_length, padding='post')
ytrain = keras.utils.to_categorical(labels, num_classes=5)

encoded_docs = tokenizer.texts_to_sequences(test_documents)
labels = test_data['category']
Xtest = pad_sequences(encoded_docs, maxlen=max_length, padding='post')
ytest = keras.utils.to_categorical(labels, num_classes=5)

Now, load the GloVe embedding to make an embedding layer. vocab_size is the number of  words in the vocabulary.

def load_embedding(filename,encoding): 
    file = open(filename,'r',encoding=encoding)
    lines = file.readlines()[1:]
    file.close()
    embedding = dict()
    for line in lines:
        parts = line.split()
        embedding[parts[0]] = asarray(parts[1:], dtype='float32')
    return embedding

vocab_size = len(tokenizer.word_index)+1
raw_embedding = load_embedding('../input/glove-global-vectors-for-word-representation/glove.6B.100d.txt','utf8')

weight_matrix = zeros((vocab_size, 100))
for word,i in tokenizer.word_index.items():
    if word in raw_embedding:
        weight_matrix[i] = raw_embedding[word]
embedding_layer = Embedding(vocab_size, 100, weights=[weight_matrix], input_length=max_length, trainable=True)

Step 4: Train the Model

vocab_size = len(tokenizer.word_index) + 1

model = Sequential()
model.add(Embedding(vocab_size, 100, input_length = max_length))
model.add(Conv1D(filters=16, kernel_size=16, activation='relu'))
model.add(MaxPooling1D(pool_size=2))
model.add(Conv1D(filters=32, kernel_size=32, activation='relu'))
model.add(MaxPooling1D(pool_size=2))
model.add(Flatten())
model.add(Dropout(0.2))
model.add(Dense(5, activation='softmax'))
print(model.summary())
model.compile(loss='categorical_crossentropy', optimizer='adam', metrics=['accuracy'])
model.fit(Xtrain, ytrain, epochs=10, verbose=2, validation_data = (Xtest,ytest))

Step 5: Make predictions on the Test set and plot the Confusion Matrix

ypred = model.predict(Xtest)
pred_labels = []
for probs in ypred:
    label = np.argmax(probs, axis=-1)
    pred_labels.append(int(label))
actual_labels = list(labels)

cm = confusion_matrix(actual_labels, pred_labels)
cmap = plt.cm.Blues
title = "Confusion Matrix"
classes = 5
normalize = False
tick_marks = np.arange(classes)
plt.imshow(cm, interpolation='nearest', cmap=cmap)
plt.title(title)
plt.colorbar()
tick_marks = np.arange(5)

fmt = '.2f' if normalize else 'd'
thresh = cm.max() / 2.
for i, j in itertools.product(range(cm.shape[0]), range(cm.shape[1])):
    plt.text(j, i, format(cm[i, j], fmt),
             horizontalalignment="center",
             color="white" if cm[i, j] > thresh else "black")

plt.tight_layout()
plt.ylabel('True label')
plt.xlabel('Predicted label')
plt.show()
Confusion Matrix for the model above

Step 6: Check the accuracy

metric = keras.metrics.CategoricalAccuracy()
metric.update_state(ytest, ypred)
metric.result().numpy()

The model we just trained has an accuracy of 90.25% on the testing set.

[Optional]

Find out the public notebook with the entire implementation below:

1-D CNN Document Classification
Explore and run machine learning code with Kaggle Notebooks | Using data from multiple data sources

[Bonus]

Checkout the below post on Machine Learning Mastery for implementation of time series forecasting using convnets:

How to Develop Convolutional Neural Network Models for Time Series Forecasting
Convolutional Neural Network models, or CNNs for short, can be applied to time series forecasting. There are many types of […]

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